The implosion which occurs upon breakage of the envelope of an evacuated cathode ray tube (CRT) is quite dangerous. Impact on the glass faceplate of such a tube can cause a faceplate to shatter into many fragments, which may be violently driven into the interior of the tube by external air pressure. The glass fragments then rebound outwardly and are ejected with sufficient force to cause serious injury to a person standing in front of the tube. Until recently, all color television tubes have consisted of CRT's with convexly curved face plates. Such faceplates resist external air pressure in much the same manner as an arch supports an architectural load, and for that reason prior art methods of implosion protection have proved adequate.
Three techniques of implosion protection were previously used with curved faceplates. In one of these, a metal band in hoop tension around the skirt of the faceplate exerts a radial compressive force which cooperates with the external air pressure to put the curve faceplate in the compression. This system is exemplified by the following U.S. Patents: Henry et al, U.S. Pat. No. 2,147,017; Vincent et al, U.S. Pat. No. 2,785,820; and Lange et al, U.S. Pat. No. 3,200,188.
These tension band systems depend upon the fact that the glass faceplate is under compression. Although brittle, glass is quite strong when it is under compression. The prior art has also been known to fasten metal strips along the side of the curved faceplate underneath the metal tension band. The metal strips redistribute the compression load applied by the tension band to the faceplate s that the load is not concentrated at the corners of the faceplate.
In a second prior art system, known as the resin bond approach, a shell is placed around the faceplate skirt and filled with epoxy. The epoxy glues the faceplate to the funnel (rear portion) of the tube to keep scattering of the glass fragments at a minimum.
Then there is a third approach, which involves securing an implosion protection panel to the front surface of the faceplate by means of an adhesive which tightly bonds the two members together to form a monolithic structure. There is a significant body of prior art disclosing the use of bonded panels in connection with curved faceplates, including the following patents:
______________________________________ U.S. Patents ______________________________________ Sumiyoshi et al. 4,031,553 Moulton et al. 2,596,863 Jackman 3,007,833 Giacchetti et al. 3,051,782 Hedler et al. 3,075,870 Kufrovich 3,113,347 Casciari 3,130,854 Anderson 3,184,327 McGary et al. 3,265,234 Applegath et al. 3,315,035 De Gier 3,422,298 Carlyle et al. 3,321,099 Lanciano 4,329,620 Arond et al. 3,208,902 Bayes et al. 3,177,090 Barnes 2,734,142 British Patents ______________________________________ Downing 875,612 Darlaston et al. 889,457 ______________________________________
But curved faceplates require that the shadow mask employed in color TV systems must also be curved. Recently, a superior color CRT has been invented which employs a flat, tensioned shadow mask and a flat faceplate, and this has resulted in a major improvement in the brightness and/or contrast of the color image.
Unfortunately, the implosion protection systems which have been used successfully with curved faceplate tubes have proven inadequate when used with flat faceplates. In particular, when prior art implosion protection systems are tested on the new flat tension mask tubes, they fail to meet UL1418, the relevant safety standard of Underwriters Laboratories, Inc. for television implosion hazards.
Attempts to use prior art implosion protection approaches with flat tension mask tubes have been unsuccessful. In particular, systems employing implosion protection panels bonded to the front of the faceplate have not proved satisfactory. High speed video tape motion pictures of test implosions of flat tension mask tubes show clearly that the entire monolithic implosion panel-and-faceplate structure disintegrates as a unit upon frontal impact, creating an abundant supply of glass fragments which are fired out the front of the tube at high velocity. The effect is a dangerous blizzard of glass shards.
The prior art also discloses the use of an external magnetic shield placed around the funnel portion of a CRT. The external magnetic shield is utilized to protect the CRT from the earth's magnetic field and thus enhance the quality of the picture. A color CRT is usually manufactured and shipped without such an external magnetic shield. Prior art methods of subsequently securing such a shield to the CRT generally involve the use of add-on fastening means near either end of the shield.
A new type of implosion protection system has recently been developed and is disclosed in the patent applications cited above. This system employs a UV-curable releasable resin formulation to bond an implosion panel to a CRT faceplate, the formulation being designed to achieve separation of the implosion panel from the faceplate upon impact.
The present invention provides a modification of the above-described implosion protection system, adding a tension band which cooperates with the previously disclosed implosion panel and resin layer system to help prevent dangerous implosions. The tension band system can also be used to secure an external magnetic shield to the CRT. It has been discovered that the alternative approach, like its predecessors, also provides an extremely reliable flat faceplate implosion protection system.
More specifically, this alternative approach contemplates the addition of a tension band assembly to a faceplate having a resin system which releasably binds an implosion panel to the faceplate. The tension band assembly includes a metal tension band and optionally also comprises metal keeper bars or strips around the CRT faceplate. The latter make it possible to secure an external magnetic shield around the CRT funnel at the time of production, and also to secure the metal tension band to the faceplate. The metal strips also redistribute the compression load applied by the tension band to the faceplate, so that the load is not concentrated at the corners of the faceplate. The result is an implosion protection system for flat-faced CRT's that is extremely reliable.